Machines for pumping material, as liquid manure, have power driven impeller pumps that deliver liquid manure under pressure to discharge pipes. Gate valves mounted in the pipes are used to control the flow of liquid manure through one or more pipes. Johnson in U.S. Pat. No. 3,905,725 shows a material control flow valve having a housing and a movable valve plate located within a chamber in the housing. Paulson et al in U.S. Pat. No. 4,284,371 shows a pair of valve assemblies associated with the outlet of a pump. Each valve assembly has a movable valving plate operably connected to a hand-operated lever. The lever is manually operable to move the valve plate between its open and closed positions. The lever is connected to a rod mounted on a housing having a chamber accommodating the valving plate. The valving plate fits between the walls of the housing so that the walls guide the plate between its open and closed positions. The valving plate has a relatively loose sliding fit with respect to the walls so that it can be moved with the hand-operated lever. When the valving plate is in the closed position, it does not seal the fluid flow passage through the valve housing. Thus, the valve assembly leaks when the valving member is closed. The amount of leakage of the fluid, as liquid manure, through the closed valve increases as the valve plate wears. In use, materials, as liquids and solids, can accumulate in the chamber between the valve plate and end wall of the housing. This material makes it difficult to completely open the valve plate and at times prevents the valve plate from being moved to its open position. The valve chamber must be drained and cleaned out to return the valve assembly to its normal operating condition. The guillotine valve assembly of the invention obviates the disadvantages of the prior gate valves.